The invention relates to a method of manufacturing a hollow cone provided with an open side and a cone tip remote from the open side, whereby a parison of viscous material is brought into a first mould part after which the parison of material is moulded into the desired shape of the hollow cone by means of a second mould part.
The invention also relates to a device suitable for carrying out the method according to the invention, which device is provided with a first and a second mould part which are displaceable relative to one another.
The invention further relates to a cone manufactured by the method according to the invention and to a cathode ray tube provided with such a cone.
The term xe2x80x9cviscous materialxe2x80x9d is understood to mean a material such as glass or synthetic resin with a viscosity of 102-107 Pa.s.
A method and a device suitable for manufacturing a cone are known from German Patent DE-C2 2734773.
During the manufacture of a cone, the parison of material is introduced into the first mould part and then pressed into the desired shape by means of the second mould part. The first mould part has a funnel shape and the parison is introduced into a comparatively narrow portion of the funnel shape. The cone tip is formed in the narrowest portion of the funnel shape during moulding. The required wall thickness in the vicinity of the cone tip of the cone to be shaped is comparatively small compared with the rest of the cone. During moulding, the material is pressed into the narrowest portion of the first mould part, from which a portion of the material is pressed towards the wider portion of the first mould part through interspacings between the first and second mould parts. During moulding, the distance between the first and second mould parts becomes increasingly smaller, so that the material is forced to flow through ever narrower interspacings towards the wider portion of the funnel shape. Furthermore, a thin skin of cooled material is formed against the mould parts during moulding. This renders the interspacings through which material can flow even narrower. Comparatively high compression forces are necessary for forcing the material through these narrow interspacings. It is possible for the cone tip to break off from the rest of the cone with these high compression forces. This breaking of the cone tip is the result of the excessive pressure on the material which has already cooled down.
A solution proposed for this problem in the known device is a better temperature control in that the wall thickness of the mould parts in the vicinity of the cone tip to be formed is at least twice as thin as the wall thickness of the remaining portions of the mould parts.
A disadvantage of the known device is that the determination of the required wall thicknesses is inconvenient because new mould parts are to be manufactured each time for changing the wall thicknesses.
The invention has for its object to provide a simple method for the manufacture of a hollow cone by which the said disadvantages are avoided.
According to the invention, the method is for this purpose characterized in that an escape space for the material becomes accessible in the vicinity of the cone tip to be formed and adjoining the first mould part the moment the pressure present in the material in the vicinity of the cone tip to be formed exceeds a previously defined value.
The material which during moulding cannot or can hardly flow through the interspacings between the mould parts towards the wider portion of the funnel shape anymore will try to escape to the escape space. The entrance to the escape space, however, is blocked until the pressure in the material in the vicinity of the cone tip exceeds a previously defined value and the escape space becomes accessible. The pressure in the material in the vicinity of the cone tip in this way never becomes higher than a predetermined value.
A blocking mechanism which is comparatively easy to provide and in which the previously defined value of the admissible pressure in the material in the vicinity of the cone tip can be easily changed in an embodiment of the method according to the invention is characterized in that the escape of the material into the escape space is prevented by means of a gas pressure applied in the escape space until the pressure present in the material has become greater than the gas pressure.
The desired gas pressure level is experimentally determined, while the gas pressure may be readily changed so as to optimize the moulding process.
An alternative embodiment of the method according to the invention is characterized in that, after the parison of material has been introduced into the first mould part, the parison is kept floating in the first mould part by means of the gas pressure until moulding starts.
The time between the moment the viscous parison of material is introduced into the first mould part and the start of the moulding process in practice is a few seconds. A cold layer arises in the parison in all locations where the parison touches the walls of the mould part, because the parison transfers heat to the colder mould part. The formation of the cold layer, especially in locations where the cone to be formed has a comparatively thin wall, means that the required compression force is comparatively great. Sagging of the parison by its own weight is prevented in that the parison is kept floating in the mould part until moulding starts, so that the contact surface area between the parison and the mould part remains comparatively small and only a comparatively small cold layer is formed before moulding starts. Owing to this measure, the required compression force is comparatively small, which contributes to the prevention of cone tip fracture.
A further embodiment of the method according to the invention is characterized in that the formed cone is pressed from the first mould part by means of the gas pressure after moulding.
The removal of the moulded cone from the first mould part is facilitated thereby.
It is noted that the use of compressed air for stripping the formed cone from the mould part is known per se from German Patent Application DE 2001977. In the method according to the invention, however, the gas pressure is used both for preventing cone tip fracture and for removing the formed cone from the mould part.
A yet further embodiment of the method according to the invention is characterized in that the gas pressure is adjustable during the manufacture of the hollow cone.
This renders it possible to adapt the gas pressure to the desired pressure at any moment during cone manufacture. The relevant pressure gradient is experimentally determined.
The invention also has for its object to provide a device for the manufacture of a hollow cone by which the disadvantages of the known device are avoided.
The device according to the invention suitable for carrying out the method according to the invention is for this purpose characterized in that an escape space with closing possibility merges into the first mould part.
The escape space is not accessible to the parison of material until the moment the pressure in the material exceeds a previously defined value. The term xe2x80x9cwith closing possibilityxe2x80x9d in relation to the escape space in this connection is accordingly understood to mean that the escape space can be blocked in such a manner that no material can escape into the escape space;
An embodiment of a device suitable for carrying out the method according to the invention, in which the escape space is initially kept inaccessible by means of gas pressure until the pressure in the material has become greater than the gas pressure, is characterized in that the escape space is provided with a gas pressure connection.
The desired gas pressure is applied in the escape space through the gas pressure connection. The use of gas for rendering the escape space temporarily inaccessible has the advantage that no moving mechanical parts need be used in and around the hot escape space (400xc2x0 C). Moving parts may cause malfunctioning and are subject to wear.
A further embodiment of a device suitable for carrying out the method according to the invention is characterized in that the escape space is funnel-shaped, a comparatively large open side of the escape space merging into the first mould part and a comparatively small open side of the escape space being connected to the gas connection.
Owing to the funnel-shaped escape space, the cone can be readily pressed from the escape space and the mould part after moulding.